Due to environmental consciousness, which has increasingly grown in the past few years, and thanks to various technical developments, it was possible to significantly reduce the amount of harmful substances in the exhaust gases of internal combustion engines. Nitrogen oxides (NO, NO2, in general NOx), a significant part of which may be reduced to nitrogen during an after-treatment of exhaust gases, for example with the aid of selective catalytic reduction, continue to be an important emission class. In order to be able to analyze and optimize the composition of exhaust gases, in particular with regard to their after-treatment, it is necessary to accurately determine the concentration of the harmful substances, e.g., of the nitrogen oxides, contained in the exhaust gas. Here, even the smallest quantities of harmful substances, e.g., in the ppm range, need to be reliably determinable.
However, detecting nitrogen oxides is not an easy task due to the high amount of oxygen also being present in the exhaust gas. For measuring small nitrogen oxide concentrations in the simultaneous presence of oxygen, ZrO2-based solid electrolyte sensors are usually used. Such a sensor is provided with multiple sections or chambers which are generally separated from each other via diffusion barriers. Here, the oxygen concentration is reduced in a first chamber to a predetermined value by a first electrolytic oxygen pump cell. The pump voltage is applied to the electrolyte in such a way that the resulting electric potential in the electrolyte drives a flow of oxygen ions out of the chamber. By selecting the physical conditions, such as the temperature, the catalytic effect of the electrode material and the variable of the applied pump voltage, it is achieved that the nitrogen oxides, as oxygenic compounds or other oxygenic compounds to be determined, are not decomposed or only a small amount of them is decomposed, and they do not contribute or only contribute to a reduced extent to the oxygen pumping flow out of the first chamber. In another chamber, another electrolytic pump cell further reduces the oxygen concentration to such an extent that the nitrogen oxides, being oxygenic compounds, are dissociated into oxygen and nitrogen. The electric current arising by the pumping of oxygen, usually ranging between nA and μA, is measured and represents a measure for the nitrogen oxide concentration in the measured gas. For pumping out the oxygen alone, one or multiple internal pump electrodes (IPE) may be provided which are situated in one or multiple first chamber(s). The other electrode, through which the decomposition of the nitrogen oxides takes place, is also referred to as a decomposition electrode (NOE).
In nitrogen oxide concentrations of only a few ppm, the amount of the electric current to be measured is usually very small, e.g., only few nA, so that even the smallest interferences have a dramatic effect. In particular, leakage currents at the sensor itself, at the sensor housing or in the cable may be erroneously interpreted as an NOx current. In order to compensate for that, a complex calibration is usually performed after manufacturing NOx sensors of this type. The calibration result is stored in a memory assigned to the sensor, usually in the sensor evaluation electronics. Over the lifetime of the sensor, however, shifts may develop, which falsify the basis value of the NOx signal and its gradient, due to various aging phenomena such as change in the internal resistance, electrode aging, temperature drift, change in the contacts/insulation resistances, etc. Furthermore, an increased burst of oxygen to the decomposition electrode may result in a change in the NOx signal. If this burst has not already occurred during the starting calibration, the burst will cause significant measuring errors during ongoing operation.
In order to prevent problems of this type, it is desirable to be able to perform in-situ recalibration of an NOx sensor during operation. This is, however, difficult because there is no independent measurement of the NOx value which might be used for calibration. The conventional nitrogen oxide sensors additionally have a cross sensitivity to NH3. During the normal driving cycle, a state never occurs in which neither NOx nor NH3 reliably prevails, so that the measuring signal might be unambiguously assigned to either NOx or NH3.
Against this backdrop, an object of the present invention is to provide a method for calibrating an NOx sensor or other sensors for determining, in particular, oxygenic gas components, which allows the sensor to be measured or calibrated during ongoing operation, so that sensor changes occurring over the operating time of the sensor may be taken into account. This object may be achieved with the aid of an example method for measuring and/or calibrating a gas sensor which is provided for determining, in particular, oxygenic gas components in gas mixtures according to the present invention.